The present invention relates to a novel composite metal material, a preparation process thereof, and a photocatalytic process using same, particularly photolysis of water effectuated with visible light to produce hydrogen and oxygen.
Many studies have been made on photon energy conversion systems to utilize solar energy effectively. In particular, there has been a great interest in producing hydrogen and oxygen from water by bringing water into contact with light in the presence of a photocatalyst. Such a process has the advantages that the resource is unlimited and clean and high-power energy can be provided without generating environmental problems.
Since A. Fujishima and K. Honda reported the use of a TiO2 electrode as a photocatalyst in Nature, Vol. 37, 238 (1972), many photocatalysts have been developed. For example, U.S. Pat. No. 5,262,023 discloses homogeneous semiconductor photocatalysts such as TiO2 and SrTiO3. Further, Catalysis Today, Vol. 44, pp 17-26 (1998) discloses the use of heterogeneous photocatalysts such as BaTi4O9, K2Ti6O13, Na2Ti6O13 and K4Nb6O17, in combination with a minor amount of an oxide of a transition, rare earth or novel metal, such as NiOx, RuOx, RhOx or Pt.
However, the photocatalysts disclosed in the prior art references exhibit marginal photoylsis activity only at the wavelength range of ultraviolet light.
As an attempt to avoid the use of UV light, S. Ikeda et al. suggested the use of CuFeO2 as a photocatalyst for photolysis of water at the wavelength range of near visible light in Stud. Surf Sci. Catal. 121, Proc. TOCAT 3, 301-304 (1999). However, this material has such a low activity that hydrogen can be produced at a rate of only about 1 xcexcmol/hr of catalyst.
Although other materials have also been reported to function as photocatalysts in photolysis of water with near visible light (e.g., see J. Yoshimura et al., Bull. Chem. Soc. Hpn., Vol. 68, 2439-2445 (1995)), they still have low activities.
It is, therefore, a primary object of the invention to provide new photocatalytic materials which are stable under photolysis conditions and have excellent activities as well as good activity maintenance characteristics in photolyzing water with visible light.
In accordance with an aspect of the present invention, there is provided a composite metal compound of formula (I)
x(A)y(B)z(C)-p(P)n(N)xe2x80x83xe2x80x83(I)
wherein,
(A) is Pb(Mg1/3Nb2/3)O3 or Pb(Zn1/3Nb2/3)O3,
(B) is PbTiO3,
(C) is LiTaO3,
(P) is a metal selected from the group consisting of Pt, Au, Ag, Pd and Rh,
(N) is an oxide of a metal selected from the group consisting of Ni, Co, Fe, Sr, Sc, Ru, Cu and Cd,
x is a number in the range of 0.65 to 0.98,
y is a number in the range of 0.01 to 0.34,
z is a number in the range of 0.01 to 0.1, and
p and n are each independently a number in the range of 0.01 to 5.
In accordance with another aspect the present invention, there is provided a process for preparing the composite metal compound of formula (I) in the form of particles, which comprise the steps of:
(a) mixing a component selected from Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 with PbTiO3, and LiTaO3, in relative molar amounts ranging from 0.65 to 0.98, 0.01 to 0.34 and 0.01 to 0.1, respectively,
(b) adding, to the mixture obtained in (a), a metal selected from the group consisting of Pt, Au, Ag, Pd and Rh, and an oxide of a metal selected from the group consisting of Ni, Co, Fe, Sr, Sc, Ru, Cu and Cd, in amounts ranging from 0.01 to 5% by weight based on the mixture, and
(c) calcining the mixture obtained in (b), followed by pulverizing the calcination product.
In accordance with a further aspect of the present invention, there is provided a single crystal of formula (I).
In accordance with a further aspect of the present invention, there is provided a process for preparing a single crystal of formula (I), which further comprises the step of
(d) melting down the compound of formula (I) obtained in the above step (c) and cooling the molten product to crystallize.
In accordance with a still further aspect of the present invention, there is provided a photocatalytic process for producing hydrogen and oxygen from water using the inventive composite metal material in the form of particles of the compound or the single crystal of formula (I) at the wavelength range of visible light.